How fast is your dinghy ?

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Boo2

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Just curious to know how fast I can expect an 8 or 9 foot dinghy to go when powered by a 3.5hp engine ? Haven't decided what dinghy to get yet but am considering a Bic 245 or maybe an inflatable.

So, how fast is your dinghy ?

Boo2
 
You are on borderline of maximum displacement speed of around 4.5 knots, but as others have said you can get some inflatables, if they have the right sort of floor, to just about plane with one (lightish) person on board. Realistically if you are using it as a tender and carrying extra weight of gear or crew member you are limited to 4-5 knots. if you want planing performance with any kind of load you need a boat and engine that become too big and heavy for a practical yacht tender unless you want to go down the route of engine derricks and davits.
 
As said the speed of a dinghy is almost entirely dependent on the weight it carries. if the load is light enough (like one light person whose mass is correctly placed in the dinghy) then it will plane. Once the dinghy is planing then a limiting factor might be the pitch of the prop and max RPM of the engine. But might also depend on HP of engine to keep it planing. In practice your loaded dinghy will be hull speed limited. It will take lots of HP to improve on that. good luck olewill
 
My Achilles inflatable, with me (110kg), easily gets up onto the plane with a 4HP outboard. But not with me plus SWMBO. In Oz there would be many with nothing less than 25HP – it is that first 5m acceleration that keeps you out of the jaws of a croc . . . (not kidding). Andrew
 
Tranona said:
You are on borderline of maximum displacement speed of around 4.5 knots, but as others have said you can get some inflatables, if they have the right sort of floor, to just about plane with one (lightish) person on board. Realistically if you are using it as a tender and carrying extra weight of gear or crew member you are limited to 4-5 knots. if you want planing performance with any kind of load you need a boat and engine that become too big and heavy for a practical yacht tender unless you want to go down the route of engine derricks and davits.
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Re the planing bit, and requiring something too big or heavy: I beg to differ. I have a 3 metre (2.9?) Zodiac Zoom RIB 9 (my yacht tender), light enough to be carried on my own: will plane with 2 adults and a little kit with 8 hp Tohatsu, quite easily.
 
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Laurie said:
Re the planning bit, I beg to differ. I have a 3 metre (2.9?) Zodiac RIB 9 (my yacht tender), light enough to be carried on my own: will plane with 2 adults and a little kit with 8 hp Tohatsu.
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There is a world of difference between your RIB plus 8hp and a dinghy with 3.5hp which the OP was asking about. Exactly the point I was making, if you want proper planing performance you need a set up like yours, which I would suggest is not practical for a cruising tender on a 30-35 ft yacht unless you have the gear to handle it.
 
The Bic 245 is quite a light dinghy (being polyethylene) and flat bottomed, in flat waters it should plane easily with 3.5hp. But in a chop or at speed it is likely to be wet as it has little freeboard. I find the same with inflatables, they throw up a lot of spray in anything but a glassy calm so speed takes second place to comfort - I hate arriving at the boat already soaked through!

Rob.
 
Tranona said:
There is a world of difference between your RIB plus 8hp and a dinghy with 3.5hp which the OP was asking about. Exactly the point I was making, if you want proper planing performance you need a set up like yours, which I would suggest is not practical for a cruising tender on a 30-35 ft yacht unless you have the gear to handle it.
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I'm not sure quite what you mean by "gear to handle it"? I have none, it is a light weight tender for two adults and kit, or more when required (then it wont plane!). It is easily lifted by one adult. The engine is on the limit of that which I would like to hike aboard without any davit...about 24kg I believe. The boat is 28 foot, but I used similar to get out to my 22 footer......
I would suggest that perhaps smaller may not be practical, but of course that depends on the environs, distance etc.? Horses for courses as always......?
 
:QUOTE=rob2;4642697]The Bic 245 is quite a light dinghy (being polyethylene) and flat bottomed, in flat waters it should plane easily with 3.5hp. But in a chop or at speed it is likely to be wet as it has little freeboard. I find the same with inflatables, they throw up a lot of spray in anything but a glassy calm so speed takes second place to comfort - I hate arriving at the boat already soaked through!

Rob.[/QUOTE]

True enough!!encouragement:[
 
Laurie said:
I'm not sure quite what you mean by "gear to handle it"? I have none, it is a light weight tender for two adults and kit, or more when required (then it wont plane!). It is easily lifted by one adult. The engine is on the limit of that which I would like to hike aboard without any davit...about 24kg I believe. The boat is 28 foot, but I used similar to get out to my 22 footer......
I would suggest that perhaps smaller may not be practical, but of course that depends on the environs, distance etc.? Horses for courses as always......?
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Fine for getting out to your mooring and maybe towing for short distances, but not really practical for longer distance cruising where the dinghy and outboard get stowed on board. Hence the popularity of full inflatables and under 4hp motors as tenders, particularly if you have a marina berth.
 
Tranona said:
Fine for getting out to your mooring and maybe towing for short distances, but not really practical for longer distance cruising where the dinghy and outboard get stowed on board. Hence the popularity of full inflatables and under 4hp motors as tenders, particularly if you have a marina berth.
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True enough..............oops second time I've said that!!
 
Our tender is a 3.1 metre Zodiac. With a 3.3 Mariner it will plane with just one on board but, for most practical purposes, it merely chugs along at around 4 knots.
 
Boo2 said:
Just curious to know how fast I can expect an 8 or 9 foot dinghy to go when powered by a 3.5hp engine ? Haven't decided what dinghy to get yet but am considering a Bic 245 or maybe an inflatable.

So, how fast is your dinghy ?

Boo2
Click to expand...
If you are worried about speed, because you have a mooring a fair way from where you launch, you might do well to look for a longer dinghy, but still as light as you can find.
If the distance is not so great, speed doesn't matter much unless you have a lot of tide to argue with.
Qualities like stability and keeping your kit/backside dry are more important.

I used to have a 14 ft mock clinker dinghy that was good for 5.5 knots, two-up and kit for a weekend with a 4hp 2T mariner.
Not too slow with a racing crew of 5 either.
Or about 4.5 knots with 2hp.

Fastest I've gone with the 4hp mariner is 13 knots timed over half a mile, but I was surfing the wake of he Police launch.....
That was in a little 8ft grp Dory, quite lightly built.

For a dinghy to take cruising, AX3 one up with 2hp probably does 5 knots, 2 up perhaps 4?
But it is much more quickly carried up the beach than anything heavier.

My current dinghy is good for 20knots under sail, in the right hands and conditions....
 
About 12 knots with a yamaha malta on my tinker foldaway. This is with just one person on board sitting as far forward as possible. In general I suspect 4 to 5 knots with 2 or 3 on board when it will not plane so limited to hull speed.
 
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